[clang-tidy] cppcoreguidelines-pro-type-member-init check flags local variables of std::array<> type

[avaneyev]

cppcoreguidelines-pro-type-member-init flags local std::array<> variables as uninitialized. This seems wrong, as these aren't members of a structure or class, where the check should be applied.
Setting the IgnoreArrays config parameter to true does not help. Not sure if this case was supposed to be covered by the IgnoreArrays param in the first place, or this is just a bug.

Example program:

include <array>

void func() {
    std::array<char, 10> someArray;
    someArray.fill('n');
}

which produces the following warning and a fix suggestion:

<source>:4:5: warning: uninitialized record type: 'someArray' [cppcoreguidelines-pro-type-member-init]
    4 |     std::array<char, 10> someArray;
      |     ^                             
      |                                   {}
1 warning generated.

(on godbolt: https://godbolt.org/z/rY9zPv7hG).

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Anton Vaneev (avaneyev)

`cppcoreguidelines-pro-type-member-init ` flags local `std::array<>` variables as uninitialized. This seems wrong, as these aren't members of a structure or class, where the check should be applied. Setting the `IgnoreArrays` config parameter to true does not help. Not sure if this case was supposed to be covered by the `IgnoreArrays` param in the first place, or this is just a bug.

Example program:

include <array>

void func() {
    std::array<char, 10> someArray;
    someArray.fill('n');
}

which produces the following warning and a fix suggestion:

<source>:4:5: warning: uninitialized record type: 'someArray' [cppcoreguidelines-pro-type-member-init]
    4 |     std::array<char, 10> someArray;
      |     ^                             
      |                                   {}
1 warning generated.

(on godbolt: https://godbolt.org/z/rY9zPv7hG).

[chrchr-github]

From the documentation:
The check also flags variables with automatic storage duration that have record types without a user-provided constructor and are not initialized. The suggested fix is to zero initialize the variable via {} for C++11 and beyond or = {} for older language versions.
So this seems like a true positive, especially since it's easy to forget that std::arraydoesn't initialize its elements like other containers do.

[zeyi2]

Thanks for reporting the issue. I'm fixing the problem right now :)

[avaneyev]

@chrchr-github

this seems like a true positive, especially since it's easy to forget that std::arraydoesn't initialize its elements like other containers do.

If this behaviour is intentional, either IgnoreArrays should turn the warning and the suggestion off, or there should be an alternative if this particular parameter is not quite applicable.
As documentation for this exact check also states,

For performance critical code, it may be important to not initialize fixed-size array members.

and we run into this case very often.
In the example above there is no fix other than turning off or suppressing the lint rule, as the array element type is a primitive built-in type and we cannot just "fix" the constructor.

[avaneyev]

Thanks for the fix @zeyi2!